Osmolality is an index of the solute concentration. It is determined by the number and not by the nature of the particles in solution. Dissolved solutes change the physical properties of solutions; they increase the osmotic pressure and decrease freezing point.

Body fluids have the same osmolality as a corresponding serum specimen taken at the same time. "True body fluids" include: ascitic, cerebrospinal, hydrocele, edema, pericardial, pleural, spermatocele and synovial fluids. Secretions not in equilibrium with the extra-cellular fluids of the body include gastric juice, saliva and sweat.

Serum osmolality is normally between 275 to 295 mOsm/kg; it increases with dehydration and decreases with overhydration. Urine osmolality reflects the ability of the kidney to maintain tonicity and water balance. The normal kidney can concentrate urine to 800 to 1,400 mOsm/kg, and with excess fluid intake, a minimal osmolality of 40 to 80 mOsm/kg can be obtained.